1. Field of the Invention
The present invention relates to a hermetically sealed electrical feed-through device, especially for an initiator or squib of an air bag igniter.
2. Description of the Related Art
Air bag systems used for protecting the passengers of a motor vehicle during a collision include an inflatable bag mounted in the dashboard or steering wheel, gas generators for the explosive generation of gas to inflate the bag, acceleration sensors to generate an electrical signal indicative of a collision and an igniter responsive to the acceleration sensors for ignition of a trigger charge in a hollow chamber that, in turn, ignites a main charge to produce the gas that inflates the bag.
The igniter for an air bag system comprises a so-called header or squib. The squib or header, as described for example in U.S. Pat. No. 5,243,492 and U.S. Pat. No. 5,772,243, comprises a hermetically sealed electrical feed-through device for supplying a current into the hollow chamber containing the trigger charge and a thin bridge wire electrically connected across the electrical feed-through device. The bridge wire ignites the trigger charge when a sufficient electrical current is passed through it via the feed-through device. Hermetically sealed electrical feed-through devices are also used for other types of devices.
In methods of manufacturing the hermetically sealed electrical feed-through devices of the prior art, as described in U.S. Pat. No. 5,709,724, U.S. Pat. No. 6,274,252, U.S. Pat. No. 5,243,492, U.S. Pat. No. 5,157,831, U.S. Pat. No. 4,678,358 and U.S. Pat. No. 4,430,376, a metal isolator body or plug is provided with a circular through-going opening or a metal eyelet is provided with a circular cavity. A conductive pin, called the isolated pin, is hermetically sealed in the through-going opening or cavity by means of a glass-to-metal seal.
Glass-to-metal seals may be of the compression variety, in which advantage is taken of the difference in the thermal expansion properties of metal and glass, or may be due to molecular bonding, as described in U.S. Pat. No. 5,709,724 and U.S. Pat. No. 6,274,252.
A hermetically sealed feed-through device is made with a glass-to-metal compression seal by cutting an appropriately sized glass preform having a suitable coefficient of thermal expansion with a central hole, arranging the glass preform in a through-going opening in a metal disk or in a cavity in a metal eyelet, inserting the conductive isolated pin in the hole in the preform, heating the assembly to an elevated temperature over the softening point of the glass perform and then cooling the entire assembly, whereby the metal disk or eyelet contracts more than the glass. A ground pin may be connected to the metal disk or the eyelet approximately parallel to the isolated pin as described in U.S. Pat. No. 5,243,492.
The isolated pin and/or the ground pin may also be provided with a noble metal coating to protect against corrosion, as described in U.S. Pat. Nos. 4,788,382 and 5,157,831.
The resulting electrical feed-through devices can be used to make the headers or squibs for the air bag igniter, for example, by connecting the bridge wire across the glass seal between the isolated pin on the front side of the electrical feed-through device and the body of the eyelet or metal disk.
The conductive pins in the prior art electrical feed-through devices are circular cross-sectioned and the through-going opening or cavity in the prior art metal ring or eyelet is circular. Disadvantageously only bridge wires of a comparatively narrow range of lengths can thus be connected across the front side of the electrical feed-through device to make a squib or initiator, e.g. for an air bag, because the isolated pin is placed centrally in the through-going opening in the metal ring or eyelet during assembly. Thus different embodiments of the electrical feed-through devices with different sized through-going openings and different diameter isolated pins must be manufactured e.g. for different air bag system manufacturers or for different initiator or squib manufacturers. This results in comparatively large manufacturing costs including storage, distribution and fixturing expenses, because of the various different types of hermetically sealed electrical feed-through devices for the different initiators.